Composite materials are increasingly being used in aircraft structures. Composite material is used in wing panels, attachments to fuel tanks of the aircraft, etc. Composite material has the advantage of being light in weight and having good strength characteristics.
A composite material such as graphite/epoxy includes a surface region made of an electrically insulating material and an inner region that has a much lower conductivity than metal at room temperature. Composite members are fastened to each other and to metal members using metal bolts. Electric charge travels on and in the composite member and arcs from the composite member to the metal fastener if the current density exceeds the current carrying ability of the joint.
Arcing from the composite member to metal is particularly dangerous when the arcing results from a lightning strike. A lightning strike may have up to 200,000 amps at 200,000 volts though it lasts for a very short period of time. When lightning strikes the aircraft, a large charge is placed on the composite member. When the charge arcs from the composite member to a metal fastener tremendous energy is expended in a short period of time. This energy includes heat energy that heats up the air around the arc. The composite member is also heated, sometimes turning small particles of the composite material into gas instantaneously. Small particles of the composite material or metal fastener are also heated to glowing hot and broken off of the composite member. The energy of the lightning strike and associated heat and pressure buildup is often sufficient to eject these small particles, in the form of glowing sparks, several feet from the arcing location.
In the event the arcing is occurring near a fuel tank, it is important to keep the sparks from igniting the fuel or any fuel vapors.
One solution is to place a dielectric barrier over the fastener and encapsulate the arcing interfaces of the fastener with the dielectric barrier, as taught in U.S. Pat. No. 4,382,049. This solution adds considerable weight to the aircraft, given the many thousands of metal fasteners. It is also very costly and time-consuming.